Most synthetic vascular grafts presently used are made of either Dacron or Teflon. Neither of these materials has provided the answer for vascular replacement of vessels less than 6mm. The objective of this study is to transfer technology on vascular prostheses made from a segmented polyether polyurethane that has unique characteristics that enable this prosthesis to stretch and pulsate like normal vessels. No material presently in use is capable of doing this. Such a system would provide for marked improvement in life expectancy as well as the quality of patient's life with both occlusive and aneurysmal vascular diseases. The major elements of this prosthesis will be its high compliance, flexibility and durability, minimal porosity, and a graft that is totally nonreactive with blood and vascular tissues. The ultimate aim of this study will be to fabricate an arterial replacement that becomes functionally integrated into the body. It will be able to function as a blood conduct and maintain its patency indefinitely. Anastomotic connections will be developed and incorporated into the ends of the prosthesis to permit a smooth transition from the cut end of the natural vessel to the synthetic prosthesis. This will provide a smooth, leak-free anastomotic junction, reduce intravascular coagulation and eliminate neointimal hyperplasia for increased short-term survival and long-term patency. Extensive physical, chemical, and animal testing of the polymers and prostheses will be done in order to assure its compatability with the host tissues. In vitro testing of vessels will be directed primarily towards strength, durability, flexibility, and compliance in test apparatuses which are specifically designed to severely stress candidate prostheses. Once the designs and fabrication processes have been proven, animal testing of prototype units will begin. (Animal testing of production units will be performed in Phase II of the project). Prostheses will be implanted in carotid arteries of sheep and goats in order to evaluate thromboresistance, blood and tissue compatibility and the stability of the anastomotic connecting mechanisms.